Electron lens



Aug. 28, 1945. K.SPANGENBERG 2,383,751

ELECTRON LENS Filed m '1, 1942 Z-Sheets-Sheet 1 INVENTOR KARLSPANGENBEEG ATTORNEY Aug. 28,1945. K. SP ANGEN BERG 2,383,751

ELECTRON LENS I I Filed May 7, 1942 2 Sheets-Sheet 2 I 2 as: 4- .56169 0'5" :0 40800010609000! INVENTOR KARL Smuaszvesea' A'I'I'OR EY niflcationand focal length.

Patented At 2.8, 1945 UNlTED STATES PATENT OFFICE ELECTRON LENS KarlSpangenberg, Palo Alto, Calif., assignor to International StandardElectric Corporation,

New York, N. Y., a corporation of Delaware Application May 7, 1942,Serial No. 442,129

' 8 Claims;

This invention relates to electron lenses and more particularly to equaldiameter cylinder lenses;

In the. past various types of electron lenses have been proposed andconstructed. These lenses have generally varied quite widely in mag- Itis generally desired to make an electron lens with the smallesttransverse dimension practicable in order that the neck portion of atube retaining the lens maybe kept small in diameter.

Furthermore, for certain types of apparatus such as velocity modulatedtubes or cathoderay tubes used for some types of scanning or commutatingpurposes, for example, it is desirable thatthe cathode beam beconcentrated by a lens having a long focal length. Such a lens serves tomaintain the beam relatively small in cross-section over along path andgenerally produces a smaller spot at the remote end thereof.

With these requirements in mind it is an object of my invention toprovide an electron lens havture opening and, therefore, whenever theaperture is enlarged the entire diameter must be quite order that theymay widely increased. Accordingly, a weak lens con-.

struction utilizing this type of concentrating arrangement has notgenerally been adopted.

The, other type of lens most commonly used, is one composed of twocylindrical electrodes. In the case of cylindrical electrodes, it hasbeen considered that the lens action takes place at a plane ofdiscontinuity of field representing the adjacent ends of thesecylinders.- when the cylinders are made of different diameters, the lensarrangement has been made so thatthe ends of the cylinders aresubstantially in the same plane. However, when equal diametercylindrical electrodes were used, a small amount of spec is required inbe insulated from one another so that diiferent potentials may beapplied thereto. According to the prior art-small spacings weretherefore provided between these cylinders, generally in the order of,about one-tenth of the cylinder diameter although the spacing was ing asmall transverse dimension and along focal length.

According to a feature of my invention these and other objects areaccomplished by providinga lens comprising two axially spaced equaldiameter cylinders, the spacing between the cylinders being relativelygreat with respect to the diameter thereof.

A better understanding of my invention and the objects and featuresthereof may be had from the particular description thereof made withreference to the accompanying drawings in which:

Fig. 1 is an illustration of a typical electron lens incorporating thefeatures of my invention, and- Fig. 2 is a graph illustrating thefeatures of my invention.

Before entering into a specific the apparatus it will help to clarifythe understanding of my invention to discuss generally the propertiesand construction of electron lens of the prior art.

Usually one of three types of electron lens systems have been generallyadopted. One of these types is called an aperture lens and consists oftwo spaced 'discs to which positive'potentials are.

applied, the discs being provided with central apertures through whichthe electron beam passes. Inzthis type of lens, the lens action isgenerally considered as taking'place in the space between the spaceddiscs and apertures. The disadvantage of this type'of lens whereversmall dimensions are required is that'the diameter of the disc must bequite large compared tothe aperdescription of varied slightly dependingupon the potential differences at which the cylinders were designed. to

be polarized. Thus, if a large potential diiferensce I was contemplateda. slightly greater spacing was producing a great spreading of theelectron beam.

required. However, all .of these spacings between cylinders weregenerally made less than a quarter of the cylinder diameter.

A third type of lens-may be mentioned as consisting of one cylinderelement and one aperture element. This type of arrangement. ofcourse,suffers from the same disadvantage of small aperture lenses since itrequires a large transverse dimension to accommodate the apertured disc.It should be realized that with a. lens made of cylinder elements thesmallest overall diameter with respect to aperture openings is provided.Accordingly, in order to retain small dimensions this is the preferabletype of lens.

I have discovered that equal diameter cylinder lenses need not be madewith the ends closely spaced but may beprovided with relatively widespacings between the cylindrical elements. I When this is done it isfound that such a lens has a much longer focal length and therefore iscapable of focusing a. beam at a distant point, without Ithas been foundthat when the spacing is increased while maintaining the same objectdistance and voltage ratio of energy applied to the cylinders of thelens, the focal length also increases. This increase in focal length isnot very great as the separation between "the cylinders is increased upto aboutv half the diameter of the lens cylinders. However, beyond thishalf diameter relationship it is found that the diilerence is quitegreat. At a separation equal to the cylinder diameters, it is found thatthe focal length of the lens is in the order of six times that of theknown forms of lens with the same object distance and voltage ratioapplied to the lens electrodes.

In Fig. 1 is illustrated a typical structural arrangement of an electronbeam producing arbe provided instead of the specific arrangementillustrated in Fig. l.

Spaced longitudinally in the direction of travel of the electron beamare arranged two equal diameter cylinders 55 and t. A positive voltageVi is applied to cylinder 5 and a positive voltage V2 is applied tocylinder 6. All of the electrodes 3, d, 5 and t are supported oninsulating rods 7 and 8, which in turn are fastened firmly to the base9, which also serves to support the cathode i and heat shield electrode2.

At 0 is shown an arrow representing the point of smallest cross-sectionof the accelerated electron beam. This may be considered as the ob jectof the lens. This object O is reproduced as an image I on the far sideof the electron lenses 5, 6. The center of the lens is indicated by thedotted line C. The distance from O to C indicated by p, is known as theobject distance and the distance q from the center of'the lens to imageI is known as the image distance. An electron lens of the type shown, orof any other type must be considered equivalent to a thick lens inoptics.

Accordingly, there are to be considered two different rays shown as mand rb in the drawings. The ray Ta proceeds from the arrow 0 at an anglecrossing the center line axis of the lens at point F. At plane P the rayra. is bent and travels in a substantially parallel ray to thecorresponding pointon the image. The point F whereray ra crosses thelongitudinal center line of the lens is known as the first principalfocus. Ray rb may be considered as starting from the point on image I toa plane known as the second p pal plane Pl, where it is bent andtraverses parallel to the lens axis to the corresponding point on objectO. The distance between the second principal plane and the point Fl,where ray 1b crosses the longitudinal center line is known as the secondprincipal focus.

In all the electron lenses where any particular structure, has beendetermined, there are four variables any two of which takes thecharacteristics of the lens. Thus, when two lens electrodes of diameterD' and spacing S are fixed in a tube structure the four remainingvariables are object distance 2), image distance q, magnification m andvoltage ratio V2 /Vl'. When any two of these parameters are fixed theothers are also fixed, and only can be used in the particular relationto produce focus. Thus, if the object and image distance are once fixed,only one voltage ratio will produce a proper focus and the magnificationis determined thereby. I have discovered, however, that a fifth element,namely the spacing S, can be used to provide an electron lens of smalldiameter which is relatively weak. This is accomplished by increasingthe spacing between the lens elements.

In Fig. 2 is shown a graph on a log-log scale demonstrating thedifference in properties of the prior art equal diameter cylinder lens,and the equal diameter cylinder lens in accordance with my invention.The solid line graph illustrams the pattern for a prior art lens inwhich the spacing between the equal diameter cylinders is equal to 0.1of the cylinder diameter. In this graph the abscissa is shown as theobject distance p and the ordinates as the image distance q. The lenspattern then takes the form of a curvilinear rectangle. Along the linesubstantially of 45 positive slope is represented the voltage ratio.

On this is the voltage ratio of V2 to Vi as indicated in Fig. l. Themagnification then follows the other side of the rectangle. As indicatedmagnifications of 0.1, 1 and ,5 are shown, the broken line graphrepresents the corresponding pattern for equal diameter cylindricallenses spaced apart a distance equal to thediameter of the cylinder. I

By reference to this graph the features of my invention may be readilydetermined. For example, at object distance it the prior art lens has animage distance of substantially 1.4:, while the lens with the 1 to ispacing has an image distance approximately 2.6 at the same voltageratio 10. At the lower voltage ratio of ,5, the image distance for thesame object distance is 4 for the prior art lens, and about 16 for the 1to 1 ratio lens. At 20 object distance the prior art lens shows an imagedistance of approximately 6.2 at a voltage ratio of 3, while the lenswith a 1 to 1 spacing has an image distance of approximately 80. a

From the above consideration anddiscussion it is clear that an electronlens constructed in accordance with my invention has a much greaterobject to image distance than is possible with the prior art type oflens. Preferably, the specing in accordance with my invention is madegreater than half the diameter of the cylinders involved and betweenthis dimension and about three times the diameter. Results have shownthat the most useful spacing considering the space requirements, as wellas the other features thereof, occurs when the spacing S is maintainedbetween the limits of about .75 to 1.25 times the diameter of thecylinders involved. Also, better results are obtained when the voltageratio VZ/Vl is between the limits of 1 to 1, and 5 to 1. However, theadvantages of my invention are not limited to' these optimum values asis clear from the examples shown in the plotted curves.

In all electron lenses I have found that the magnification may beexpressed by the formula where K is a factor determined by the type oflens. In the known form of lens of the. prior art,

this factor Kis generally in the order. of .8. Howof the presentinvention, with its open end cylinders, differs widely from the priorart devices using apertured barrier electrodes, even where in some casesthe apertures in the barrier electrodes are lined with tubes, since suchtubes not only do not serve the purpose of the cylinders used in thecylinder type of, electron lens, in bringing to a junction at the imageplane all those trajectories of the electrons which intersect at thesame point in the object area, but serve chiefly to aid in trapping andbarring from the aperture those electrons whose trajectories make toogreat an angle with the axis of the aperture tubes.

It may be said that in all electron lens devices employing cylinders, inthe sense in which that term is used in this art, the internal diameterof the end of the cylinder which first receives the electron beam is solocated and of such size that if the tangents, to the trajectories ofall the electrons emerging from the emission aperture of the beamforming device, be determined at the plane of the emission aperture andbe considered as' extended 'in the direction of'theelectron flow, allsuch extensions of said tangents will intersect the plane of the openend of the cylinder nearest the emission aperture within thecircumference of said open end of the cylinder. In other words, all theelectrons emitted from the emission aperture can, without the action ofany deviating force, enter and pass into the open end of the firstcylinder, so that thereafter both cylinders enclose the paths ofsubstantially all electrons emitted by the emission device.

This is entirely difierent from the aperture type of electron lens, evenwhere the apertures are provided with tubes, since such apertures andtubes are much smaller than the cylinders and, hence, the devices trapand hold back all those electrons whose extended trajectory tangents,determined at the plane of the emission aperture, do not intersect theplane of the aperture, or of the open end of its aperture tube, withinthe circumference of such aperture or tube, unless some additionaldeviating force such as a strong magnetic field be applied to theelectrons in advance of the first aperture tube.

An electron lens in accordance with my invention in which the spacing ismade equal to the diameter of the lens element, has been found to ofsaid open end within the circumference thereof said cylinders beingspaced apart in the direction of electron flow a distance between oneand three times said diameter, and means for continuously maintainingpositive potentials on said cylinders, the ratio of the potentials onthe two cylinders, taken in the direction of electron flow, being notless than one and not more than ten. r

2. An electron supply means for producing an electron beam withrelatively small spot size, comprising an electron emitting element,means for accelerating and concentrating electrons from said electronsupply means into an electron beam, said means having an emissionaperture from which the electrons of said beam emerge and an electronlens in the path-of said beam comprising two open ended conductivecylinders of the same diameter arranged end to end in the path of saidelectron beam, the open end near-1 est the emission aperture, of thefirst cylinder, being arranged so that the tangents, atthe plane oi theemission aperture, to the trajectories of all the emerging electrons, ifextended in the direction of electron flow would intersect the plane ofsaid open end within the circumference thereof said cylinders beingspaced apart in the direction of electron flow a distance at least asgreat as said diameter and means for continuously "maintaining positivepotentials on said cylinders.

3. An electron supply means for producing an electron beam withrelatively small spot size; comprising an electron emitting element,means for accelerating and concentrating electrons from said electronsupply means into an electron beam, said means having an emissionaperture from which the electrons of said beam emerge and an electronlens in the path of said beam .comprising two open ended conductivecylinders of the same diameter arranged end to-end in the path of saidelectron beam, the open end nearest 1 the emission aperture, of thefirst cylinder, being in general have a magnification about 20% belowthat of all known prior art lens, and the focal length at least sixtimes as great.

While Ihave described above the principal features of my invention, itis considered that this is given merely by way of example. Certaindepartures from my inventionmay be had-within the scope of t einvention.

What I claim is; 1;. An electron supply means for producing an electronbeam with relatively small spot size, comprising an electron emittingelement, means for accelerating and concentrating electrons from saidelectron supply means into an electron beam,

said means having an emission aperture from, which the electrons'oi saidbeam emerge and an theemerging. electrons, if-extended in thedirectionoi electron flow wouldintersect the plane arranged so that thetangents, at the plane of the emission aperture, to the trajectories ofall the emerging electrons, if extended in the direction oi electronflow would intersect the plane of said open end within the circumferencethereof said cylinders being spaced apart in the direction of electronflow a distance between one and three times said diameter, and means forcontinuously maintaining on said cylinders positive potentials of suchratio, taken in the direction of electron flow as to give the desiredspot size at the image plane.-

4. An lectron lens for controlling the focus of an electron beam ofelectrons emerging at I various angles from an emission aperture toprovide a relatively long focal length, comprising a pair of open endedcylindrical elements located substantially coaxially in the path of thebeam and arranged to receive all the electrons directly from theemission aperture over the paths they were following when emerging fromsaid emission aperture, said elements being of substantially equaldiameter andlongitudinally spaced apart a distance equal to at leastabout said diameter and means for maintaining positive potentials onsaid'elements.

5. An electron lens according to claim 4 where-.- in the ratio of thepotential applied to the cylindrical element furthest in the directionof electron flow of said beam and that applied to the other element isnot less than one and not more I 6. An electron lens according to claim4 in which the ratio of the potential applied'to the cylinder furthestin the direction of electron trons directly from the emission aperture,over,

the paths they were following when emerging from the emission aperture,said elements being of substantially equal diameter and longitudinallyspaced'apart a distance which is about the same'as the diameter of saidcylinders, and means for maintaining on each cylinder 2. positivepotential, the ratio of the potentials, taken in the direction ofelectron flow, being not less than one and not more than five.

8. An electron supply means'for producing a beam with relatively smallspot size at the image plane and with a relatively long focal length,

1 path of said beam and comprising two openended conductive cylinders ofthe same diameter, arranged end to end in the path of said beam, theopen end, nearest the emission aperture, of the first cylinder, beingarranged so that the tangents, at th plane of the emission aperture, tothe trajectories of all the emerging electrons, if extended in thedirection of the electron flow, would intersect the plane of said openend within the circumference thereof, said cylinders being spaced apart,in the direction of electron flow, a distance not less than equal to onediameter of the cylinders and not more than three times said diameter,and means for continuously inaintaining a positive potential on eachcylinder, the ratio of the potentials, taken in the .direction ofelectron flow, being not less than one and not more than fifteen.

KARL SPANGENBERG.

